Water-soluble triphenylphosphane-3,3′,3″-tricarboxylate (m-TPPTC) ligand and methylated cyclodextrins: A new combination for biphasic rhodium-catalyzed hydroformylation of higher olefins

Article abstract of DOI:10.1002/adsc.200606160

The methylated cyclodextrins/m-TPPTC [tris(m-carboxyphenyl)phosphane trilithium salt] couples proved to be more efficient than the well-known methylated cyclodextrins/TPPTS [tris(m-sulfonatophenyl)phosphane trisodium salt] systems in terms of activities and selectivities to perform the Rh-catalyzed hydroformylation of higher olefins in an aqueous-organic system. The interactions cyclodextrins/m-TPPTC have been fully studied by NMR spectroscopy.

Full text of DOI:10.1002/adsc.200606160

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DOI: 10.1002/adsc.200606160
Water-Soluble Triphenylphosphane-3,3’,3’’-tricarboxylate
m-TPPTC) Ligand and Methylated Cyclodextrins: A New
(
Combination for Biphasic Rhodium-Catalyzed Hydroformylation
of Higher Olefins
a,
b
a
c
SØbastien Tilloy, * Emilie Genin, FrØdØric Hapiot, David Landy,
c
b
b,
Sophie Fourmentin, Jean-Pierre GenÞt, VØronique Michelet, *
and Eric Monflier
Laboratoire de Physico-Chimie des Interfaces et Applications, FRE CNRS 2485, FacultØ des Sciences Jean Perrin,
UniversitØ d’Artois, rue Jean Souvraz, S.P.18-62307 Lens CØdex, France
a
a
Fax : (+33)-3-21-791-755; e-mail: sebastien.tilloy@univ-artois.fr
Laboratoire de Synth›se SØlective Organique et Produits Naturels, Ecole Nationale SupØrieure de Chimie de Paris,UMR
b
c
7573, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
Fax : (+33)-1-44-071-062; e-mail: veronique-michelet@enscp.fr
Laboratoire de Synth›se Organique et Environnement, UniversitØ du Littoral, 145 avenue Maurice Schumann, 59140
Dunkerque, France
Received: April 4, 2006; Accepted: May 29, 2006
Abstract: The methylated cyclodextrins/m-TPPTC
to increase the solubility of higher olefins, the use of
chemically modified b-cyclodextrins such as methylat-
ed cyclodextrins preserves some economical viabili-
[
tris(m-carboxyphenyl)phosphane trilithium salt]
couples proved to be more efficient than the well-
known methylated cyclodextrins/TPPTS [tris(m-sul-
fonatophenyl)phosphane trisodium salt] systems in
terms of activities and selectivities to perform the
Rh-catalyzed hydroformylation of higher olefins in
an aqueous-organic system. The interactions cyclo-
dextrins/m-TPPTC have been fully studied by
NMR spectroscopy.
[
4]
ty. Indeed, the methylated cyclodextrins that are
cheap, non-toxic and bulk commercially available
compound allowed us to achieve the hydroformyla-
tion of higher olefins with high reaction rate and che-
moselectivity, while avoiding the formation of an
emulsion and the partition of the rhodium catalyst be-
[
5]
tween the organic and aqueous phases. However, it
was found that the regioselectivity, i.e., the linear to
branched aldehydes ratio can be decreased due to in-
teraction between the TPPTS ligand and the cyclo-
Keywords: cyclodextrin; hydroformylation; phos-
phane; rhodium; water-soluble ligand
[
6]
dextrin. In this context, it was of great interest to
evaluate in a cyclodextrin-based hydrofomylation pro-
cess the behavior of rhodium complexes associated to
the highly water-soluble ligand m-TPPTC [tris(m-car-
Rhodium-catalyzed hydroformylation in an aqueous- boxyphenyl) phosphane trilithium salt] (Scheme 1), a
organic two-phase system appears to be an economi- carboxylated analogue of the sulfonated TPPTS
cal and safe approach.[1] Indeed, an aqueous biphasic ligand. Actually, while m-TPPTC has been widely
[7]
[
8]
system allows quantitative recycling of the catalyst used in organometallic reactions, its performances
and decreases harmful emissions and costs associated have never been described in the hydroformylation of
with solvent recycling. Unfortunately, the industrial olefins and the behavior of this phosphane ligand to-
viability of the biphasic hydroformylation process has wards CDs has never been investigated. We describe
only been demonstrated in the case of lower olefins herein our preliminary results concerning the m-
such as propene and butene with a Rh/TPPTS TPPTC/CDs couple and the catalytic activity of the
[
TPPTS=tris(m-sulfonatophenyl)phosphane trisodi- rhodium/m-TPPTC system in the hydroformylation of
[2]
um salt] catalytic system. In the case of higher ole- 1-octene, 1-decene and 1-dodecene.
fins (five or more carbon atoms), the olefin solubility
The interaction between m-TPPTC and native or
is too low for industrially important rates to be ach- randomly methylated a-cyclodextrin or b-cyclodextrin
ieved and the presence of a mass transfer promoter is (Scheme 1) has been studied by NMR spectroscopy.
[
3]
required. Among the different approaches proposed First, as no chemical shift was detected on the
Adv. Synth. Catal. 2006, 348, 1547 – 1552
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1547

SØbastien Tilloy et al.
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TPPTC complex. This was consistent with a decrease
in the association constant between the CD and the
phosphane which was generally observed when in-
creasing the temperature. The apparent association
constants of these adducts were evaluated by a com-
petition method with methyl orange (MO) from UV-
VIS spectroscopic data.
The association constant values were found to be
À1
1
2
923M
for the 1:1 b-CD/m-TPPTC complex, and
À1
56M for the 1:2 b-CD/m-TPPTC complex. The be-
havior of the b-CD/m-TPPTC supramolecular com-
plex then appeared to be very different from that ob-
served for the b-CD/TPPTS couple for which a 1:1
stoichiometry was measured and an association con-
À1
[9]
stant of 1200M was calculated. The natural ten-
dency of carboxylates to form dimeric species in solu-
tion might be an explanation for the presence of 1:2
complexes in the case of the m-TPPTC ligand.
Accordingly, T-ROESY spectra were recorded to
determine whether the 1:1 and 1:2 b-CD/m-TPPTC
adducts were true inclusion complexes or adducts re-
sulting from external interaction phenomena. The T-
Scheme 1.
1H NMR spectra of a-CD/m-TPPTC or Rame-a-CD/ ROESY spectrum of a b-CD/m-TPPTC mixture for
m-TPPTC mixtures at various concentrations, it was which concentrations have been chosen to maximize
concluded that a-CDs did not interact with the phos- the 1:1 complex, evidenced the presence of correla-
phane. By contrast, native b-CD and m-TPPTC tion peaks between the aromatic protons and inner
strongly interacted together as evidenced by NMR protons of b-CD, indicating that guests parts were in-
spectroscopy. In order to determine the stoichiometry cluded in its cavities. An important part of the con-
of the complex in solution, we plotted the Jobꢁs dia- tour plot obtained by this sequence is displayed in
grams based on the induced chemical shifts of select- Figure 2.
ed protons in m-TPPTC and b-CD (Figure 1) as well
as the induced chemical shift of the m-TPPTC phos-
phorus atom.
At 5 and 258C, the Jobꢁs diagrams showed a maxi-
mum at host/(host+guest) ratio of 0.40 consistent
with a mixture of 1:1 and 1:2 b-CD/m-TPPTC com-
plexes. By contrast, at 808C, the Jobꢁs plot were much
more symmetrical and indicative of a 1:1 b-CD/m-
Figure 1. Continuous variation plots (Jobꢁs plot) derived
1
from the H NMR data for the b-CD/m-TPPTC system:
&
=
Figure 2. 2D T-ROESY NMR spectrum of a 1:1 b-CD/m-
H-1, ~ =H-3 (258C).
TPPTC complex in D O (258C).
2
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Biphasic Rhodium-Catalyzed Hydroformylation of Higher Olefins
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Table 1. Biphasic rhodium-catalyzed hydroformylation of higher olefins with TPPTS or m-TPPTC as hydrosoluble ligand
[a]
and RAME-b-CD or RAME-b-CD as mass transfer promoter
[b]
[c]
[d]
Entry
Olefin
Ligand
Cyclodextrin
Conversion [%]
Selectivity [%]
l/b ratio
1
2
3
4
5
6
7
8
9
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-decene
1-decene
1-decene
1-decene
1-decene
1-decene
1-dodecene
1-dodecene
1-dodecene
1-dodecene
1-dodecene
1-dodecene
m-TPPTC
TPPTS
m-TPPTC
TPPTS
m-TPPTC
TPPTS
m-TPPTC
TPPTS
m-TPPTC
TPPTS
m-TPPTC
TPPTS
m-TPPTC
TPPTS
m-TPPTC
TPPTS
-
-
94
2
87
45
92
93
95
97
86
59
95
91
96
98
83
72
83
85
85
83
2.3
2.8
2.0
3.0
1.5
1.7
2.4
2.8
2.0
2.8
1.5
1.8
2.3
2.8
2.1
2.8
1.6
1.7
RAME-a-CD
RAME-a-CD
RAME-b-CD
RAME-b-CD
-
100
32
100
91
43
2
80
21
100
90
16
1
-
RAME-a-CD
RAME-a-CD
RAME-b-CD
RAME-b-CD
-
10
11
12
13
14
15
16
17
18
-
RAME-a-CD
RAME-a-CD
RAME-b-CD
RAME-b-CD
68
5
88
31
m-TPPTC
TPPTS
[
a]
Experimental conditions: Rh(acac)(CO) : 1.1 mmol; ligand: 5.5 mmol; CD: 13 mmol; water: 0.32 mL; olefin: 550 mmol; n-
A
H
R
U
G
2
undecane (internal standard): 50 mmol; P(CO/H : 1/1)=40 bar; T=808C; time: 3 h.
Olefin conversion.
Aldehyde selectivity.
Linear to branched aldehydes ratio.
2
[
[
[
b]
c]
d]
Intense correlations peaks between the Ho and Hm with m-TPPTC were notably higher (multiplied by a
protons of the phosphane with the inner protons H-3 factor of 47, 21 and 16 for 1-octene, 1-decene and 1-
and H-5 of the CD were indicative of an inclusion of dodecene, respectively). The surface activity of m-
one of the aromatic rings of the phosphane in the CD TPPTC might be responsible for its higher efficiency
cavity. As the correlation peak was much more in- in the hydroformylation reaction. As anticipated
tense with H-3, the inclusion process seemed to occur (Figure 3), interfacial tension measurements per-
by the secondary face of the CD. A correlation peak formed with m-TPPTC and TPPTS clearly revealed
between Hp and H-5 also revealed the closeness be- two different behaviors: TPPTS is a hydrotropic com-
tween them. The proton Hoꢀ correlated with H-3 and pound whereas m-TPPTC behaves as a surface active
H-5 in a less intense way than the other phosphane compound. At the ligand concentration used in the
protons. Starting from these observations, the geome- catalytic experiments (17 mM), the surface tensions
À1
À1
try of the inclusion complex could be deduced as a (ST) were 42 mNm and 62 mNm
for m-TPPTC
bent phosphane in the CD cavity. Interestingly, a simi- and TPPTS, respectively.
lar orientation has been described in the case of
It should be pointed out that the selectivities in al-
dehydes in the absence of CDs were also higher with
TPPTS-b-CD inclusion complex.[
10]
The behavior of m-TPPTC was then studied in the m-TPPTC ligand than with TPPTS ligand, whereas
hydroformylation of 1-octene, 1-decene and 1-dode- the regioselectivities were slightly lower (entries 1 and
cene in a biphasic system using Rh
A
H
R
U
G
alyst precursor and methylated CDs (Rame-a-CD or compared to m-TPPTC might be advocated to explain
[
8e]
Rame-b-CD) as mass transfer promoters. For compar- such behaviors.
Indeed, Fell et al. have shown that
ison, the same experiments have been performed with in the presence of a ligand possessing a stronger p-
TPPTS as a ligand. The results are summarized in acidity, a higher selectivity to linear aldehyde was ob-
[
11]
Table 1. Without CD and with TPPTS as a ligand, the served.
A recent determination of the basicity of
conversions were very low whatever the olefin due to the carboxylated phosphane m-TPPTC showed that
the low mass transfer between the two layers (en- this ligand seems to be more electron-rich, e.g., less
[
8e]
tries 2, 8, 14). By contrast, the conversions obtained acid than its sulfonated analogue.
This feature had
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1549

SØbastien Tilloy et al.
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One way to explain this result could be a CD-induced
decoordination process during the coordination of the
olefin/Rame-a-CD inclusion complex on the rhodium
center to reduce steric hindrance around the metal. A
similar hypothesis was recently invoked to explain the
l/b ratio decrease in the hydroformylation of 1-decene
with
a
Rame-a-CD/alkyl-sulfonated diphosphane
combination for which no inclusion complex was de-
[
12]
tected.
The cyclodextrins/m-TPPTC interactions have thus
been studied and were found to be very peculiar. The
methylated CDs/m-TPPTC couples therefore showed
interesting features for Rh-catalyzed hydroformyla-
tion reactions. The methylated CDs/m-TPPTC sys-
tems proved to be more efficient than the well-known
methylated CDs/TPPTS ones in terms of activities
and selectivities in the hydroformylation of higher
olefins. Experiments are currently under way to refine
our understanding of the catalytic process to explain
the loss of regioselectivity observed with the Rame-a-
CD.
Figure 3. Surface tension of TPPTS (^) and m-TPPTC (+)
curves in water at 208C.
already some consequences on the Rh-catalyzed CÀC
bond formation, where the ligand m-TPPTC showed Experimental Section
a very high efficiency and selectivity compared to
TPPTS.
[
8a,c–e]
General Remarks
When methylated CDs were used as mass transfer
promoters, the conversions were increased whatever
the ligand or the olefin (compare entries 9 and 11
with 7, for example). Actually, the inclusion of olefins
in the hydrophobic cavity of the CDs led to the for-
mation of host/guest complexes at the liquid/liquid in-
terface that greatly enhanced the reaction rate. Using
methylated CDs also improved the chemoselectivity
1
The H NMR spectra were recorded on a Bruker Avance
DRX spectrometer. The 2D T-ROESY experiments were
run using the software supplied by Bruker. T-ROESY ex-
periments were preferred to classical ROESY experiments
as this sequence provides reliable dipolar cross-peaks with a
minimal contribution of scalar transfer. Mixing times for T-
ROESY experiments were set at 300 ms. The data matrix
for the T-ROESY was made of 512 free induction decays,
due to the protective effect of the CD cavity, which 1 K points each, resulting from the co-addition of 32 scans.
compelled the substrate to react in a more controlled The real resolution was 1.5–6.0 Hz/point in F2 and F1 di-
way (compare entries 3 and 5 with 1, for example). It mensions, respectively. They were transformed in the non-
phase-sensitive mode after QSINE window processing. UV-
is worth noting that, once the olefin was coordinated
VIS spectroscopy was performed on a Perkin–Elmer Lamba
to the rhodium, an interaction of these CDs with the
2
5 spectrometer. The cell used was placed in a cuvette
catalytic species could not be excluded to explain the
chemoselectivity increase. Contrary to activities and
chemoselectivities, the regioselectivities of the hydro-
formylation reaction strongly depended on the nature
of the ligand and the CDs. With TPPTS as ligand, the
holder and the temperature was kept constant at 298 K
Æ0.1 by means of a thermostated bath. The interfacial ten-
sion measurements were performed using a KSV Instru-
ments digital tensiometer (Sigma 70) with a platinum plate.
The precision of the force transducer of the surface tension
À1
linear to branched aldehydes ratios (l/b) measured in apparatus was 0.1 mNm.
The experiments were per-
the presence of Rame-a-CD were rather close or formed at 293 K Æ0.5 controlled by a thermostated bath
identical to those observed without CD whereas a de- Lauda (RC6 CS). Gas chromatographic analyses were car-
ried out on a Shimadzu GC-17 A gas chromatograph equip-
crease in the l/b ratios was observed for Rame-b-CD.
ped with
a methylsilicone capillary column (30 m ”
By contrast, with m-TPPTC as ligand, the l/b ratio
was decreased in the presence of both methylated
CDs though only Rame-b-CD was able to trap m-
TPPTC. As the l/b ratio decrease observed with the
CD derivatives has been attributed in previous works
to the formation of inclusion complexes between the
0
.32 mm) and a flame ionization detector (GC:FID). D O
2
(
99.95% isotopic purity) was obtained from Merck. Dicar-
bonylacetylacetonatorhodium(I) was purchased from Al-
drich Chemicals in its highest purity and used without fur-
ther purification. Organic compounds (undecane, 1-octene,
1
-decene and 1-dodecene) were purchased from Aldrich
ligand and the cyclodextrin, this phenomenon with Chemicals in their highest purity and used without further
the Rame-a-CD and m-TPPTC is clearly unexpected. purification. Rame-a-CD was prepared by adapting a proce-
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Adv. Synth. Catal. 2006, 348, 1547 – 1552

Biphasic Rhodium-Catalyzed Hydroformylation of Higher Olefins
COMMUNICATIONS
dure reported by Y. Kenichi et al.[13] Rame-b-CD was pur-
chased from Aldrich Chemicals. TPPTS and m-TPPTC were
[
14]
prepared as reported in the literature.
hydrogen mixtures (1:1) were used directly from cylinders
>99.9% pure; Air Liquide). Distilled deionized water was
Carbon monoxide/
(
used in all experiments. All liquid reagents were degassed
by bubbling nitrogen for 15 min before each use or by two
freeze-pump-thaw cycles before use.
This concentration is then used to calculate a theoretical
value of the total concentration of m-TPPTC ([m-
TPPTC] ), by postulating values for the various expected
T
formations constants. Three kinds of inclusion compounds
have been investigated (K , K and K12 being, respectively,
1
1
21
the formation constantsof the 1:1, 2:1 and 1:2 complexes be-
tween b-CD and m-TPPTC):
Continuous Variation Plots (Jobꢀs Plot)
i) 1:1 complex:
The NMR measurements for the Jobꢁs plot were taken on
11 samples. The series of samples containing a variable ratio
(
from 0 to 1) of b-CD and m-TPPTC was prepared keeping
the total concentration of species constant (10 mm in this
1
present case). The differences of chemical shift in H NMR
spectra were measured as a function of the molar ratio.
Calculation of Association Constant
The characterization of the b-CD-m-TPTTC complexes by
the competitive method is realized in two steps. First of all,
the inclusion compound between b-CD and MO (methyl
orange) is characterized by the well-known titration
ii) mixture of 1:1 and 2:1 complexes:
method, leading to a formation constant K equal to
À1
2
500M . In a second time, various amounts of m-TPPTC
are added to solutions containing a fixed composition of b-
CD and MO, leading to various values of absorbance. In the
following, b-CD-MO represents the molecular complex be-
tween b-CD and MO, while b-CD/MO stands for a solution
of b-CD and MO; by analogy, b-CD/MO/m-TPPTC stands
for the solutions containing b-CD, MO and m-TPPTC. The
absorbance of an aqueous MO solution is described by the
iii) mixture of 1:1 and 1:2 complexes:
Beer–Lambert law: AMO =e ”[MO]T.
MO
When cyclodextrin is added, there is a modification of the
absorbance:
Ab-CD/MO =e ”[MO]+eb-CD-MO ”[b-CD-
MO
MO]. The absorbance of the pure b-CD-MO complex is
thus described by Ab-CD-MO =eb-CD-MO ”[MO]T.
When m-TPPTC, which does not absorb in the studied
region, is added, the previous relations are still valid, but
the [b-CD-MO] concentration is weaker, consecutively to
the formation of complexes between b-CD and m-TPPTC:
with
Ab-CD/MO/m-TPPTC =e ”[MO]+eb-CD-MO ”[b-CD-MO].
MO
The amount of dissociated b-CD-MO depends not only on
the concentration of added substrate, but also on the stabili-
ties and stoichiometries of the complexes thus formed. Such
perturbation of the equilibrium in the presence of m-
TPPTC may be described by the concentration of free b-
CD, which is calculated from the recorded absorbance by
the following relation:
For each case, the difference between the theoretical and
experimental values of [m-TPPTC] is then minimized by a
T
home-made algorithmic procedure. The stoichiometry induc-
ing the weaker difference between theory and experiment is
then supposed to be the correct one and the corresponding
formation constants are thus known.
Adv. Synth. Catal. 2006, 348, 1547 – 1552
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SØbastien Tilloy et al.
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T-ROESY Experiment
H. Shinokubo, K. Oshima, Angew. Chem. Int. Ed. 2005,
4
4, 2397; c) K. Kinoshita, H. Shinokubo, K. Oshima, J.
Am. Chem. Soc. 2003, 125, 7784, and references there-
in.
The T-ROESY spectrum was conducted with [m-TPPTC]=
2.5 mM and [b-CD]=7.5 mM in order to favor the 1:1 com-
plex, even if the exclusive formation of this compound is un-
achievable. For such concentrations and using the formation
constants calculated by the UV-VIS competition method,
[3] B. E. Hanson, in: Aqueous-Phase Organometallic Catal-
ysis, 2nd edn., (Eds.: B. Cornils, W. A. Herrmann),
VCH, Weinheim, 2004, p. 243.
2
8% of b-CD and 83% of m-TPPTC are complexed within
the 1:1 stoichiometry, while the 1:2 inclusion concerns only
% of b-CD and 9% of m-TPPTC.
[4] For a recent review on the applications of cyclodex-
trins, see: a) F. Hapiot, S. Tilloy, E. Monflier, Chem.
Rev. 2006, 106, 767; b) E. Monflier, G. Fremy, Y. Casta-
net, A. Mortreux, Angew. Chem. Int. Ed. 1995, 34,
1
2
269; c) E. Monflier, S. Tilloy, G. Fremy, Y. Castanet,
A. Mortreux, Tetrahedron Lett. 1995, 36, 9481.
[5] T. Mathivet, C. MØliet, Y. Castanet, A. Mortreux, L.
Caron, S. Tilloy, E. Monflier, J. Mol. Catal. A.: Chem.
2001, 176, 105.
[6] E. Monflier, H. Bricout, F. Hapiot, S. Tilloy, A.
Aghmiz, A. M. Masdeu-Bultó, Adv. Synth. Catal. 2004,
346, 425.
[7] m-TPPTC=tris(m-carboxyphenyl)phosphane trilithium
salt: R. Amengual, E. Genin, V. Michelet, M. Savignac,
J.-P. Genet, Adv. Synth. Catal. 2002, 344, 393.
Catalytic Experiments
All catalytic reactions were performed under nitrogen using
standard Schlenk techniques. A stainless steel, 150 mL auto-
clave, equipped with a carousel containing 8 vessels with
Teflon stirring bar, was used. In a typical experiment, each
vessel was charged with Rh
A
H
R
U
G
2
(TPPTS or m-TPPTC) (5.5 mmol) and CD (13 mmol) dis-
solved in 0.32 mL of water and the organic phase composed
of olefin (550 mmol) and undecane (50 mmol – GC internal
standard). The autoclave was pressurized with 40 atm of
[
8] a) R. Amengual, V. Michelet, J.-P. GenÞt, Tetrahedron
Lett. 2002, 43, 5905; b) R. MØtivier, R. Amengual, I.
Leray, V. Michelet, J.-P. GenÞt, Org. Lett. 2004, 6, 739;
c) E. Genin, V. Michelet, J.-P. GenÞt, Tetrahedron Lett.
CO/H (1/1). The mixture was stirred for 3 h at 808C. The
2
reaction medium was sampled after the reaction for GC
analyses of the organic phase after decantation.
2
004, 45, 4157; d) E. Genin, V. Michelet, J.-P. GenÞt, J.
Organomet. Chem. 2004, 689, 3820; e) E. Genin, R.
Amengual, V. Michelet, M. Savignac, A. Jutand, L.
Neuville, J.-P. GenÞt, Adv. Synth. Catal. 2004, 346,
Acknowledgements
1
733; f) K. Kranjc, M. Kocevar, F. Iosif, S. M. Coman,
This work was supported by the Centre National de la Re-
cherche Scientifique (CNRS). E.G. is grateful to the Minist›re
de l’Education et de la Recherche for financial support
V. I. Parvulescu, E. Genin, J.-P. GenÞt, V. Michelet,
Synlett 2006, 1075.
[9] E. Monflier, S. Tilloy, C. MØliet, A. Mortreux, S. Four-
mentin, D. Landy, G. Surpateanu, New. J. Chem. 1999,
23, 469.
(
2003–2006). The authors are grateful to G. Crowyn for
NMR analysis.
[
[
10] E. Monflier, S. Tilloy, L. Caron, J. M. Wieruszeski, G.
Lippens, S. Fourmentin, D. Landy, G. Surpateanu, J.
Incl. Phenom. 2000, 38, 361.
11] B. Fell, G. Papadogianakis, J. Prakt. Chem. 1994, 336,
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[
[
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TPPTC see ref.
1552
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